Stabilized contact lenses

ABSTRACT

Contact lenses with stabilization zones are designed using mathematical constructs such as Bezier curves and which are subjected to modeling on-eye performance.

BACKGROUND OF THE INVENTION

Correction of certain optical defects can be accomplished by impartingnon-spherical corrective aspects to one or more surfaces of a contactlens such as cylindrical, bifocal, or multifocal characteristics. Theselenses must generally be maintained at a specific orientation while onthe eye to be effective. Maintenance of the on-eye orientation of a lenstypically is accomplished by altering the mechanical characteristics ofthe lens. Prism stabilization including decentering of the lens' frontsurface relative to the back surface, thickening of the inferior lensperiphery, forming depressions or elevations on the lens' surface, andtruncating the lens edge are examples of stabilization approaches.Additionally, dynamic stabilization has been used in which the lens isstabilized by the use of thin zones, or areas in which the thickness ofthe lens' periphery is reduced. Typically, the thin zones are located attwo regions that are symmetric about either the vertical or horizontalaxis of the lens from the vantage point of its on-eye placement.

Evaluating lens design involves making judgments concerning theperformance of the lens on-eye and then optimizing the design ifnecessary and possible. This process is typically done by clinicallyevaluating the test design in patients. However, this process is timeconsuming and expensive because it requires a significant number ofpatients to be tested since patient to patient variability must beaccounted for.

There is a continuing need for improving the stabilization of certaincontact lenses and the method of designing them.

SUMMARY OF THE INVENTION

The invention is a method of designing stabilized contact lens in whichthe stabilization zones are defined by a mathematical construct. Theconstruct can be a Bezier curve.

In one aspect of the invention lenses are designed using coefficients ofBezier curve describing the upper portion of the angular thicknessprofile such that the Sag values are negative. When the stabilizationzone is added to the lens periphery the thickness of the upper portionof the lens is reduced instead of being increased; reducing thethickness in the upper portion of the stabilization allows reducing themaximum thickness and still keeping the same thickness differential.Slopes around the location of maximum thickness are not too muchaffected with this profile change.

In another aspect of the invention the area containing negative sagvalues is applied on the upper and lower portion of the stabilizationzones.

In another yet another aspect of the invention the maximum thickness ofthe stabilization zones differs between the left and the right side.

In yet another aspect of the invention the ramp of the thickness profiletoward the positive and/or the negative angles can be adjusted toincrease or decrease the ramp angle.

In yet another aspect of the invention lenses made according to thedesign method have improved stabilization.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a front or object view of a stabilized contact lens.

FIG. 2 is a schematic representation of an eye with an inserted lens; itidentifies the axis of rotation and various torques acting at the lens.

FIG. 3 is a Lens Thickness Map and Graph of Thickness Profiles forExample 1

FIG. 4. is a Lens Thickness Map and Graph of Thickness Profiles forExample 2

FIG. 5. Is a Lens Thickness Map and Graph of Thickness Profiles forExample 3

FIG. 6. Is a Lens Thickness Map and Graph of Thickness Profiles forExample 4

DETAILED DESCRIPTION

Contact lenses of this invention have designs that optimizestabilization based on balancing various forces that act on the lenses.This involves the application of a design process that balances torquesoperating on the eye, components of the eye, and ultimately thestabilized lens that is placed on the eye. Preferably, improvedstabilization is attained by starting the improvement process with anominal design that includes stabilization elements. For example, a lensdesign that has two stabilization zones that are symmetric about boththe horizontal and vertical axes running through the center is aconvenient reference from which to optimize stabilization of the lensaccording the inventive methods. By “stabilization zone” is meant anarea of the peripheral zone of the lens which has thickness valueslarger than the remaining areas of the peripheral zone. Thestabilization zones of the invention may in some respects have thicknessvalues that are less than the average thickness of the peripheral zoneof the lens but they will nevertheless in other respects have thicknesszones that are larger as well. By “peripheral zone” is meant the area ofthe lens surface circumferentially surrounding the optic zone of thelens, and extending up to but not including the edge of the lens.Another stabilization design that is a useful starting point isdescribed in US Patent Publication 20050237482 which is incorporatedherein by reference but any stabilization design can be used as thenominal design that is then optimized according to this invention. Thestabilization design improvement process can also incorporate testingthe improvement with the eye model described below, evaluating theresults of the testing, and continuing iteratively with the improvementprocess until a desirable level of stabilization is achieved.

FIG. 1 depicts the front, or object side, surface of a stabilized lens.Lens 10 has an optical zone 11. The lens' periphery surrounds optic zone11. Two thick regions 12 are located in the periphery and arestabilization zones.

The model that is preferably used in the process to produce the newdesigns incorporates various factors and assumptions that simulate themechanical operation and their effects on lens stability. Preferably,this model is reduced to software using standard programming and codingtechniques according to well-known programming techniques. In broadoverview, the model is used in the process for designing stabilizedlenses by simulating the application of the forces described below in aprescribed number of blinks of the eye. The degree to which the lensrotates and decenters is determined accordingly. The design is thenaltered in a way that is directed to bringing rotation and/or centrationto more desirable levels. It is then subjected to the model again todetermine translation upon blinking after the pre-determined number ofblinks.

The model assumes that the eye preferably consists of at least twospherical surface parts representing the cornea and the sclera and thatthe origin of the x-y-z coordinate axes is in the center of the sphererepresenting the cornea. More complex surfaces such as aspheric surfacesmay also be used. The base shape of the lens consists of sphericalsurface parts but the base curve radius of the lens is allowed to changefrom the center of the lens towards the edge. More than one base curvemay be used to describe the back surface. It is assumed that a lenspositioned on the eye assumes the same shape as that of the eye. Thethickness distribution of the lens need not necessarily be rotationallysymmetric and indeed is not symmetric according to some preferredembodiments of the inventive lenses. Thick zones at the edge of the lensmay be used to control the position and orientation behavior of thelens. A uniform thin film of liquid (tear film) exists between the lensand the eye, with a typical thickness of 5 μm. This tear film isreferred to as the post-lens tear film. At the lens edge the thicknessof the liquid film between the lens and eye is much smaller and isreferred to as the mucin tear film. A uniform thin film of liquid (also,tear film) with a typical thickness of 5.0 μm exists between the lensand the lower and upper eyelids and these are referred to as thepre-lens tear films. The boundaries of both the lower and the uppereyelids lie in planes having unit normal vectors in the x-y plane.Hence, the projections of these boundaries on the plane perpendicular tothe z-axis are straight lines. This assumption is also made during themotion of the eyelids. The upper eyelid exerts a uniform pressure on thecontact lens. This uniform pressure is exerted on the whole area of thecontact lens covered by the upper eyelid or on a part of this area nearthe boundary of the upper eyelid with uniform width (measured in thedirection perpendicular to the plane through the curve describing theedge of the eyelid). The lower eyelid exerts a uniform pressure on thecontact lens. This pressure is exerted on the whole area of the contactlens covered by the lower eyelid. The pressure exerted by the eyelids onthe contact lens contributes to the torque acting at the lens through anon-uniform thickness distribution (thick zone) of the contact lens,especially near the edge. The effect of this pressure on the torquesacting at the contact lens is referred to as the melon seed effect.Viscous friction exists in the post-lens tear film if the lens moveswith respect to the eye. Viscous friction also exists in the mucin tearfilm between lens edge and the eye if the lens moves with respect to theeye. Additionally, viscous friction exists in the pre-lens tear film ifthe lens moves and/or the eyelids move. Strains and stresses in the lensoccur due to the deformation of the lens. These strains and stressesresult in an elastic energy content of the lens. As the lens moves withrespect to the eye and the deformation of the lens changes, the elasticenergy content changes. The lens tends toward a position at which theelastic energy content is minimal.

The parameters describing the geometry of the eye (cornea and sclera),base shape of the lens and the motion of the eyelids the eyelids areshown in FIG. 2. The motion of the lens follows from the balance ofmoment of momentum acting at the lens. Inertial effects are neglected.Then the sum of all moments acting at the lens is zero. Hence,

{right arrow over (0)}={right arrow over (M)}_(l,cor) +{right arrow over(M)} _(l,muc) +{right arrow over (M)} _(l,low) +{right arrow over (M)}_(l,upp) +{right arrow over (M)} _(l,Ulow) +{right arrow over (M)}_(l,Uupp) +{right arrow over (M)} _(l,Vupp) +{right arrow over (M)}_(m,slow) +{right arrow over (M)} _(m,supp) +{right arrow over (M)}_(elast) +{right arrow over (M)} _(grav)

The first 4 moments are resisting torques and are linearly dependent onthe lens motion. The remaining torques are driving torques. This balanceof moment of momentum results in a non-linear first order differentialequation for the position β of the lens

${{A\left( {\overset{\_}{\beta},t} \right)}\frac{\overset{\rightarrow}{\beta}}{t}} = {{\overset{\rightarrow}{M}}_{total}^{driving}\left( {\overset{\_}{\beta},t} \right)}$

-   -   This equation is solved with a fourth order Runge-Kutta        integration scheme. The positions of points on the contact lens        follow from a rotation around the rotation vector β(t). The        rotation matrix R(t) transforming the old position of points to        the current position follows from Rodrigues's formula

${\overset{\rightarrow}{x}}_{new} = {{\overset{\rightarrow}{x}}_{old} + {\sin \; {\beta \left( {\overset{\rightarrow}{n} \times {\overset{\rightarrow}{x}}_{old}} \right)}} + {\left( {1 - {\cos \; \beta}} \right)\left( {\overset{\rightarrow}{n} \times \left( {\overset{\rightarrow}{n} \times {\overset{\rightarrow}{x}}_{old}} \right)} \right)}}$${\overset{\rightarrow}{x}}_{new} = {{R(t)}{\overset{\rightarrow}{x}}_{old}}$where$\overset{\rightarrow}{n} = {{\frac{\overset{\rightarrow}{\beta}}{\overset{\rightarrow}{\beta}}\mspace{14mu} {and}\mspace{14mu} \beta} = {{\overset{\rightarrow}{\beta}}.}}$

-   -   In the numerical integration method a time-discretization is        used. Then the motion of the lens can be seen as a number of        subsequent rotations, hence at the next time step t_(n+1) the        rotation matrix is

R _(n+1) =R _(Δt) R _(n)

-   -   where R_(Δt) is the rotation during the time step Δt.    -   The rotation matrix is decomposed into a rotation R_(α) and a        decentration R_(θ) of the lens

R(t)=R _(θ)(t)R _(α)(t)

-   -   The rotation of the lens is a rotation around the centerline of        the lens. The decentration is a rotation around a line in the        (x, y) plane. Hence, the position of the lens is seen as a        rotation {right arrow over (α)} of the lens around its        centerline followed by a decentration {right arrow over (θ)}.

The designs are made or optimized using the model described above bydescribing the design details using one or more mathematical constructs.Preferably, stabilization zones are described using Bezier curves butother mathematic descriptions can be used to get a full description ofthe stabilization zones. When the Bezier curve approach is used, aradial function A_(r)(t_(r)) describing the radial thickness profile isdefined preferably using five control. An angular function B_(α)(t_(α))describing the angular thickness profile is also defined using fivecontrol points. For example, the mathematical description can beformulated as follows:

A _(r,x)(t _(t))=P _(r1)(x).(1−t _(t))⁴+4.P _(r2)(x).(1−t _(t))³ .t_(t)+6.P _(r3)(x).(1−t _(t))² .t _(t) ²+4.P _(r4)(x).(1−t _(t)).t _(t) ³+P _(r5)(x).t _(t) ⁴

A _(r,y)(t _(t))=P _(r1)(y).(1−t _(t))⁴+4.P _(r2)(y).(1−t _(t))³ .t_(t)+6.P _(r3)(y).(1−t _(t))² .t _(t) ²+4.P _(r4)(y).(1−t _(t)).t _(t) ³+P _(r5)(y).t _(t) ⁴

Where P_(ri)(x) and P_(ri)(y) are the coordinates of the control pointsand t_(r) the normalized coordinate along the radial profile. Thestarting point that describes the radial thickness profile is defined byP_(r1) and the ending point is defined by P_(r5).

B _(α,x)(t _(α))=P _(α1)(x).(1−t _(α))⁴+4.P _(α2)(x).(1−t _(α))³ .t_(α)+6.P _(α3)(x).(1−t _(α))² .t _(α) ²+4.P _(α4)(x).(1−t _(α)).t _(α) ³+P _(α5)(x).t _(α) ⁴

B _(α,y)(t _(α))=P _(α1)(y).(1−t _(α))⁴+4.P _(α2)(y).(1−t _(α))³ .t_(α)+6.P _(α3)(y).(1−t _(α))² .t _(α) ²+4.P _(α4)(y).(1−t)t _(α) .t _(α)³ +P _(α5)(y).t _(α) ⁴

Where P_(αi)(x) and P_(αi)(y) are the coordinates of the control pointsand t, the normalized coordinate along the angular profile. The startingpoint that describes the angular thickness profile is defined by P_(α1)and the ending point is defined by P_(α5).

The magnitude of the stabilization zone described by C(t_(r), t_(α)) (3)is obtained from the product of the radial function A_(r,y) by theangular function B_(α,y). A scaling factor M is applied to the productof the two functions to control the magnitude of the stabilization zone.

C(t _(r) ,t _(α))=M.A _(r,y)(t _(r)).B _(α,y)(t _(α))

These equations and can be extended for any number of control points. Inthat case the equations can be rewritten as:

$X,{Y = {\sum\limits_{i = 1}^{N}\; {C_{i}\left( {{P_{{Xi},{Yi}}\left( {1 - t} \right)}^{N - i}t^{i - 1}} \right)}}}$C₁ = 1$C_{i} = \frac{\left( {N - 1} \right)!}{{i!}{\left( {N - i} \right)!}}$

A different set of functions can be used to describe the rightstabilization zone from the left giving an asymmetrical stabilizationzone design.

In a preferred embodiment of the invention the coefficients of Beziercurve describing the upper portion of the angular thickness profile areset such that the Sag values are negative. In that particular case whenthe stabilization zone is added to the lens periphery the thickness ofthe upper portion of the lens is reduced instead of being increased.FIG. 5 shows the effect of reducing the thickness in the upper portionof the stabilization zone. It allows reducing the maximum thickness andstill keeping the same thickness differential. Slopes around thelocation of maximum thickness are not much affected with this profilechange.

Preferably, the invention is used to design and then manufacturestabilized toric lenses or toric multifocal lenses as, for example,disclosed in U.S. Pat. Nos. 5,652,638, 5,805,260 and 6,183,082 which areincorporated herein by reference in their entireties.

As yet another alternative, the lenses of the invention may incorporatecorrection for higher order ocular aberrations, corneal topographicdata, or both. Examples of such lenses are found in U.S. Pat. Nos.6,305,802 and 6,554,425 incorporated herein by reference in theirentireties.

The lenses of the invention may be made from any suitable lens formingmaterials for manufacturing ophthalmic lenses including, withoutlimitation, spectacle, contact, and intraocular lenses. Illustrativematerials for formation of soft contact lenses include, withoutlimitation silicone elastomers, silicone-containing macromers including,without limitation, those disclosed in U.S. Pat. Nos. 5,371,147,5,314,960, and 5,057,578 incorporated in their entireties herein byreference, hydrogels, silicone-containing hydrogels, and the like andcombinations thereof. More preferably, the surface is a siloxane, orcontains a siloxane functionality, including, without limitation,polydimethyl siloxane macromers, methacryloxypropyl polyalkyl siloxanes,and mixtures thereof, silicone hydrogel or a hydrogel, such as etafilconA.

Curing of the lens material may be carried out by any convenient method.For example, the material may be deposited within a mold and cured bythermal, irradiation, chemical, electromagnetic radiation curing and thelike and combinations thereof. Preferably, for contact lens embodiments,molding is carried out using ultraviolet light or using the fullspectrum of visible light. More specifically, the precise conditionssuitable for curing the lens material will depend on the materialselected and the lens to be formed. Suitable processes are disclosed inU.S. Pat. No. 5,540,410 incorporated herein in its entirety byreference.

The contact lenses of the invention may be produced by any convenientmethod. One such method uses an OPTOFORM™ lathe with a VARIFORM™attachment to produce mold inserts. The mold inserts in turn are used toform molds. Subsequently, a suitable liquid resin is placed between themolds followed by compression and curing of the resin to form the lensesof the invention. One ordinarily skilled in the art will recognize thatany number of known methods may be used to produce the lenses of theinvention.

The invention will now be further described with respect to thefollowing non-limiting examples.

Example 1

A contact lens for astigmatic patients having a known design and whichwas designed using conventional lens design software with the followinginput design parameters was obtained:

-   -   Sphere power: −3.00D    -   Cylinder Power: −0.75D    -   Cylinder Axis: 180 deg    -   Lens diameter: 14.50 mm    -   Front optical zone diameter of 8.50 mm    -   Back optical zone diameter of 11.35 mm    -   Lens base curve: 8.55 mm

The stabilization zone is an extra thick zone added to the thicknessprofile of that lens. The left and right stabilization zones areconstructed using a set of control points (Table 1) applied to thepreviously described mathematical functions. The Lens thickness profileis shown in FIG. 3.

TABLE 1 Control points applied to example 1. Left stabilization zoneRight stabilization zone Radial Ctrl points Angular Ctrl points RadialCtrl points Angular Ctrl points Point 01 X 4.250 120 4.250 −100 Y 0.0000.000 0.000 0.000 Point 02 X 5.500 197 5.500 −33 Y 0.050 −0.025 0.050−0.025 Point 03 X 6.600 205 6.600 −25 Y 0.480 0.750 0.480 0.750 Point 04X 6.930 213 6.930 −17 Y 0.200 −0.025 0.200 −0.025 Point 05 X 7.175 2907.175 60 Y 0.000 0.000 0.000 0.000 Scaling factor 3.641 3.641

Example 2

The lens described in Example 1 had the radial location of thestabilization zones pushed out by 0.25 mm such that the optic zonediameter was extended to 9.00 mm for the selected prescription.

The left and right stabilization zones were constructed using a set ofcontrol points shown in Table 2 applied to the previously describedmathematical functions. The upper portion of the stabilization zonethickness was reduced instead of being increased. The toric contact lenshas an optic zone equivalent to what is usually offered with aconventional single vision lens. Modeling of the centration and rotationof the lens using the eye model described above showed the performanceof the lens was not significantly affected by the relocation of thestabilization zones. The Lens thickness profile is shown in FIG. 4.

TABLE 2 Control points applied to example 2. Left stabilization zoneRight stabilization zone Radial Ctrl points Angular Ctrl points RadialCtrl points Angular Ctrl points Point 01 X 4.550 120 4.550 −110 Y 0.0000.000 0.000 0.000 Point 02 X 5.500 192 5.500 −28 Y 0.050 −0.200 0.050−0.025 Point 03 X 6.650 205 6.650 −25 Y 0.470 0.800 0.470 0.800 Point 04X 6.930 208 6.930 −12 Y 0.200 −0.025 0.200 −0.200 Point 05 X 7.175 2907.175 60 Y 0.000 0.000 0.000 0.000 Scaling factor 3.3 3.3

Example 3

The lens described in Example 1 was redesigned using the method of theinvention such that the magnitude of the left stabilization zone wasreduced by 40 microns. The left and right stabilization zones wereconstructed using a set of control points as shown in Table 3 applied tothe previously described mathematical functions.

The introduction of dissymmetry in thickness requires a different designfor left eye and right in order to keep same rotation performance onboth eyes. The results from the eye model show better rotationperformance of such designs when the thickest stabilization zone has torotate from an upper to a lower position. The Lens thickness profile isshown in FIG. 5.

TABLE 3 Control points applied to example 3. Left stabilization zoneRight stabilization zone Radial Ctrl points Angular Ctrl points RadialCtrl points Angular Ctrl points Point 01 X 4.250 115 4.250 −105 Y 0.0000.000 0.000 0.000 Point 02 X 5.500 187 5.500 −23 Y 0.050 −0.200 0.050−0.025 Point 03 X 6.600 200 6.600 −20 Y 0.480 0.800 0.480 0.800 Point 04X 6.930 203 6.930 −7 Y 0.200 −0.025 0.200 −0.200 Point 05 X 7.175 2857.175 65 Y 0.000 0.00 0.000 0.000 Scaling factor 2.966 3.641

Example 4

The lens design of example 1 was modified so that the magnitude of theleft stabilization zone was reduced by 40 microns. The left and rightstabilization zones were constructed using a set of control points shownin Table 4 applied to the previously described mathematical functions.The upper and lower portion of the stabilization zone thickness wasreduced instead of being increased, reducing the thickness in the upperand lower portions of the stabilization zones and reducing the maximumthickness while retaining a similar thickness differential. The Lensthickness profile is shown in FIG. 6.

TABLE 4 Control points applied to example 4. Left stabilization zoneRight stabilization zone Radial Ctrl points Angular Ctrl points RadialCtrl points Angular Ctrl points Point 01 X 4.250 105 4.250 −75 Y 0.0000.000 0.000 0.000 Point 02 X 4.750 170 4.750 −10 Y −0.010 −0.250 −0.010−0.250 Point 03 X 4.750 180 4.750 0 Y 0.400 0.950 0.400 0.950 Point 04 X5.500 190 5.500 10 Y 0.220 −0.250 0.220 −0.250 Point 05 X 6.700 2556.700 75 Y 0.230 0.000 0.230 0.000 Point 06 X 7.050 7.050 Y 0.500 0.500Point 07 X 7.160 7.160 Y −0.010 −0.010 Point 08 X 7.175 7.175 Y 0.0000.000 Scaling factor 3.641 3.641

Utilizing the eye model described herein, lenses from Examples 1. 2, and3 show optimum rotation speed around the 40-50 degrees misalignmentrange. Designs from these examples are preferred for lenses with opticsthat depend on lens orientation such as custom vision correction lenseswhere the lens orientation is unidirectional due to the asymmetry of thestabilization zones along the horizontal axis. Those lenses also presenthigher rotation speed for lens orientations within 20 degrees from thefinal position compared to conventional marketed lenses. Furthercustomization can be obtained from example 3 where the left and rightstabilization zones are asymmetric. These designs and lenses presentgreater rotation speed for lens orientation within 30 degrees from thefinal position (relative to the existing commercial lenses).

The reduction of thickness in the stabilization zone did not affect lensperformance in rotation when the magnitude of the thickness differentialwas kept as shown with examples 1 and 2 where the magnitude of thestabilization has been reduced by 10% from example 1 to example 2. Thelens design of example 2 has a maximum stabilization zone thicknessreduced by about 20% compared to conventional products making the lensmore comfortable to the wearer.

Modeling of the lens of Example 4 showed slower rotation speed but lessrotation speed variation across the lens orientation. The design fromexample 4 is preferred for lenses with optics that does not depend onlens orientation such as toric lenses where the lens orientation can bebidirectional due to the symmetry kept in the design of thestabilization zones.

1. A contact lens stabilized with a design made by providing a lensdesign with a nominal set of stabilization zone parameters improved bycharacterizing the lens design parameters as a mathematical construct,modeling the design with a model that balances moments of momentum, andselecting the design based on the results of the modeling.
 2. A contactlens made according the method of claim 1 wherein the construct is aBezier function.
 3. The contact lens of claim 1 wherein the angularprofile of stabilization zone is defined by one or more sections withnegative thickness.
 4. The contact lens of claim 3 wherein the maximumnegative thickness value along the angular profile containing maximumthickness of the stabilization zones is between 0.010 mm to 0.060 mm andpreferably around 0.025 mm.
 5. The contact lens of claim 4 wherein thethickest portion of the stabilization zone is located between 15 degreesand 35 degrees below the horizontal axis of the lens.
 6. The contactlens of claim 5 wherein the thickest portion of the stabilization zoneis located between at least 25 degrees below the horizontal axis of thelens
 7. A contact lens of claim 1 having stabilization zones that differin thickness from one another.
 8. The contact lens of claim 7 whereinthe thickness difference between the stabilization zones is from 0.020mm to 0.045 mm.
 9. The contact lens of claim 8 wherein the thicknessdifference between the stabilization zones is at least 0.030 mm.
 10. Thecontact lens of claim 1 having stabilization zones of variable thicknesswithin each zone.
 11. The contact lens of claim 10 wherein said zoneshave a nonuniform slope from their peak to their thinnest dimension. 12.A contact lens having stabilization zones wherein the angular profile ofthe stabilization zones is defined by one or more sections with negativethickness.
 13. The contact lens of claim 12 wherein the maximum negativethickness value along the angular profile containing maximum thicknessof the stabilization zones is between 0.010 mm to 0.060 mm andpreferably around 0.025 mm.
 14. The contact lens of claim 4 wherein thethickest portion of the stabilization zone is located between 15 degreesand 35 degrees below the horizontal axis of the lens.
 15. The contactlens of claim 5 wherein the thickest portion of the stabilization zoneis located between at least 25 degrees below the horizontal axis of thelens
 16. A contact lens having stabilization zones that differ inthickness from one another.
 17. The contact lens of claim 16 wherein thethickness difference between the stabilization zones is from 0.020 mm to0.045 mm.
 18. The contact lens of claim 17 wherein the thicknessdifference between the stabilization zones is at least 0.030 mm.
 19. Acontact lens having stabilization zones having stabilization zones ofvariable thickness within each zone.
 20. The contact lens of claim 19wherein said zones have a nonuniform slope from their peak to theirthinnest dimension.